In the field of semiconductors, carrier generation and recombination are processes by which mobile charge carriers (electrons and electron holes) are created and eliminated. Recombination lifetime refers to how long the generated charge carriers, such as photocarriers in photovoltaic (PV) devices, last before returning to an energy neutral state.
Recombination lifetime, also referred to as carrier lifetime, is one of the most critical diagnostic parameters for predicting the performance of PV devices. Each product technology has specific recombination lifetime requirements with respect to the production of efficient PV converters. The dominant PV products being sold in the marketplace are typically based on silicon wafers produced by different growth methods. For residential applications, the industry compromises efficiency while maximizing the efficiency-to-cost ratio. The dominant loss mechanism that affects low-cost silicon photovoltaics is the impurity-related or defect-related Shockley-Read-Hall (SRH) recombination.
Efficiency and performance of PV devices are heavily dependent on the wafer material's photocarrier recombination lifetime, which itself is a very strong function of material purity, growth, and post-growth processing. Recombination lifetime is highly dependent on impurities and defects within a photoconducting material. For example, photocarriers induced in a sample can recombine at sites of impurities or defects in a material, thereby decreasing the photocarrier lifetime and producing unwanted heat within the material.
Thus, the recombination lifetime of photocarriers in a photoconducting sample is a useful parameter for determining the purity of the wafer material and subsequent efficiency of the PV device. A significant component of the cost of manufacturing photovoltaic materials is the loss associated with processing an inferior wafer material and consequently having to discard an entire production run because of substandard performance. Therefore, measurement of the recombination lifetime of a sample is useful as a quality control measure of a material, provided the measurement is accurate, fast and carried out in a non-invasive manner.
There are a number of techniques that are currently in use for contactless measurements of photocarrier recombination lifetime in photovoltaic materials. For example, U.S. Pat. No. 5,406,214, issued Apr. 11, 1995 to Boda et al discloses a microwave-based technique and is directed to a contactless apparatus and method for measuring minority carrier recombination lifetime in semiconducting materials using a tuned source of microwave energy. However, microwave-based techniques for measuring recombination lifetimes are currently limited to silicon materials, and although microwave reflection techniques have the ability to measure lifetimes in the nanosecond range, they have a very small dynamic range [See Ahrenkiel et al., Solar Energy materials and Solar Cells, 92, 830-835, 2008].
Similarly, quasi-steady state photoconductivity (QSSPC) techniques, such as that described by Sinton et al. [Appl. Phys. Lett., 69, 2510, 1996], are designed for measuring recombination lifetimes of silicon only and may not apply to other photovoltaic materials.
Another recombination lifetime measurement technique involves time-resolved photoluminescence (TRPL) [See e.g., Metzger et al., Journal of Applied Physics, V 94, no. 5, 3549-55, 2003; and Ahrenkiel, “Minority Carriers in III-V Semiconductors: Physics and Applications,” Semiconductors and Semimetals, Willardson Beer Series, Academic Press, V 39, 39-150, 1993], which is capable of measuring recombination lifetimes of compound semiconductor thin films in most cases. However, this technique is only applicable to direct bandgap materials and is not applicable to silicon, germanium, and other indirect bandgap materials.
U.S. Pat. No. 5,929,652, issued Jul. 27, 1999 to Ahrenkiel discloses an apparatus for determining the minority carrier lifetime of a semiconducting sample using an oscillator providing a high frequency voltage signal to a bridge circuit having a wire coil that produces a variable mutual impedance between a sample and the coil. The technique disclosed in this patent is herein referred to as “Resonance Coupled Photoconductive Decay” (“RCPCD”), and is further disclosed in U.S. Pat. Nos. 6,275,060 and 6,369,603, and Ahrenkiel et al. [Materials Science and Engineering, B102, 161-172, 2003]. The RCPCD technique is applicable to silicon wafers and films, and is useful for materials with lifetimes larger than about 30-50 ns. However, this technique may not be able to measure lifetimes in the shorter lifetime ranges that are commonly found in compound semiconductor materials and thin film silicon.
As recombination lifetime is a significant component of production yield and product cost, manufacturers of solar cells and other PV products would benefit from the ability to measure the recombination lifetime of every wafer or film prior to incorporating it an assembly line. However, there has previously been no diagnostic means available that allows for quick analysis of recombination lifetime applicable to a wide range of materials. A compromise has been reached that involves sampling photoconductive wafers at an economically practical rate in an attempt to minimize defective products, and stopping the assembly line when the material quality falls below specification. Therefore, there is a need for an improved high speed, non-invasive means for measuring the recombination photocarrier lifetime of a material, having a large dynamic range and an improved response to short-lifetime materials (such as materials having carrier lifetimes less than 50 ns), applicable to a wide range of photoconducting and semiconducting materials (such as materials used in the microelectronic and optoelectronic industries, as well as in photovoltaic industries).